RF plasma processes are employed, for example, in etching, coating and ashing procedures. Such processes have been used up to now to process relatively small surfaces. The surfaces to be processed in plasma processes, however, become larger and larger, for example larger wafers in the semiconductor industry or larger flat panel displays. It has been relatively easy up to now to generate a homogeneous plasma over the surface to be processed, but the dimensions of the surfaces to be coated or etched today reach the wavelength range of the excitation signals of the plasma or the harmonics of the excitation signals. This produces wave structures in the plasma, which makes homogeneous plasma processing difficult.
To counteract this development, several RF plasma generators are conventionally used, which are operated at the same or different frequencies. U.S. Pat. No. 5,698,062 discloses an RF generator drive means that has a low-voltage source and a frequency divider. The frequency divider drives two RF amplifiers, which may also be considered as RF generators. These supply RF power to the plasma process at different locations. U.S. Pat. No. 5,824,606 discloses driving two RF generators with the same frequency and supplying RF power to the plasma process at different locations via these RF generators. The phase positions of the signals emitted by the RF generators are adjusted relative to each other using a phase shifter.
In FPD production experiments, it is currently tried to excite plasma with MF (10 kHz to 1 MHz) energy. Several electrode pairs are disposed in the direct vicinity of other electrode pairs. Each electrode pair is supplied by an MF generator. The MF generators are not operated synchronously and not necessarily at the same frequency, which can produce high voltages and beatings between two neighboring electrode pairs. These high voltages can produce highly undesired arcs between neighboring electrode pairs.
Improved apparatus and methods for driving RF power generators are needed, particularly methods that permit flexible adjustment and control of the power output of the RF power generators or the plasma process.